In general, odorants are added to fuel gases, such as liquefied natural gas (LNG), city gas, and LP gas, to prevent gas poisoning, ignition, explosion, or other accidents caused by fuel gases, and to enable immediate and easy detection of fuel-gas leakage by emitting an offensive smell.
Sulfur-containing compounds are widely known as odorants used for these fuel gases, but have problems in that they usually generate sulfur dioxide when the fuel gases are burned. In addition, when the fuel gases are used in fuel cells, which are being developed, a desulfurizer should be installed to remove odorant components that would cause catalyst poisoning. Various non-sulfur substances are also known as odorants. They include, for example, a mixture of valeric acid and ethyl acrylate (see patent document 1), cyclohexene (see patent document 2), an odorant essentially containing 5-ethylidene-2-norbornene (see patent document 3), a combined odorant of 2-methoxy-3-iso-butylpyrazine as a non-sulfur component and a mercaptan or a sulfide (see patent document 4), and pyrazine (see patent document 5).
However, these odorants have disadvantages in that, for example, acrylic ester odorants are chemically unstable, and that the content of cyclohexene or ethylidene norbornene is larger than those of mercaptans.
Alternatively, 5-ethylidene-2-norbornene and 2-alkoxy-3-alkylpyrazine are proposed as non-sulfur fuel-gas odorants with excellent odor characteristics (see patent document 6). However, 5-ethylidene-2-norbornene has a large perceptual threshold (about 4 ppb), and 2-alkoxy-3-alkylpyrazine has nitrogen atoms, wherein the perceptual threshold is defined in the present invention.
On the other hand, with the recent development of fuel cells, the variety of fuel gases has increased; for example, methane gas, propane gas, butane gas, LNG, and hydrogen gas. In addition, a larger number of applications are being developed for fuel gases. Accordingly, there is a need for novel fuel-gas odorants, especially for non-sulfur odorants.
(Patent document 1)
    Japanese Unexamined Patent Application Publication No. 48-79804(Patent document 2)    Japanese Unexamined Patent Application Publication No. 54-58701(Patent document 3)    Japanese Unexamined Patent Application Publication No. 55-56190(Patent document 4)    Japanese Unexamined Patent Application Publication No. 60-92396(Patent document 5)    Japanese Unexamined Patent Application Publication No. 55-59190(Patent document 6)    Japanese Unexamined Patent Application Publication No. 8-60167